And regarding the discussions about what would happen in the case Instawallet's service was to be discontinued here's what we would do :

A notice would be posted a long time in advance,

We would generate a private key for each account, in a publicly documented way, using the wallet URL as seed,

We'd compute the public key from the private key,

We'd compute the address from the public key,

We'd send the balance to this generated address,

And that's it!

Now all you'd need to regain control of your coins is to follow the steps using your wallet key, you'll get a private key you can import into any client, or into any service.

Even a user without any technical knowledge could use a third-party service that could perform these steps given a wallet URL, for a fee obviously, but in perfect market conditions, therefore the fee would always be as competitive as possible.

Thoughts ?

That's smart.

It is very smart and only possible with bitcoin. You can go offline without people loosing access to their coins

I think this is the most noble of all possible solutions. With your solution coins people forgot will really be lost (until computers can brute force those by then weak keys). I guess I would have a problem seeing lost coins and count on x% never asking back their money. Most likely you could even get an insurance for that x% risk.